1. Field of the Invention
The present invention pertains to the manufacture of semiconductor devices and more particularly to passivating and protective coatings for silicon semiconductor devices.
2. Description of the Prior Art
It is known in the art that the behavior of a semiconductor device is mainly determined by the charge ratios at the edge (or peripheral surface) of the semiconducting body. In that locality, the free valences present can lead to an expansion of the space charge zone (or depletion layer) and hence to a reduction in the field strength at the edge. When blocking occurs, unsaturated valences of an opposite polarity cause a channel to form in which very high blocking currents flow.
In the past many attempts have been made to develop protective or passivating coatings whereby it is possible to eliminate, or at least neutralize, the adverse conditions which occur on the peripheral edge surface of the semiconductor body. Thus, for example, it is known in the art to employ a layer consisting of silicon dioxide (SiO.sub.2) containing a predetermined percentage of aluminum oxide (Al.sub.2 O.sub.3). By employing such a layer, it is possible to control the charge ratio conditions on the peripheral edge surface of a semiconductor body in order to raise the blocking (or inverse) voltage.
However, the magnitude of the inverse voltage is only one of several important parameters of a quality semiconductor device. Thus, for certain applications semiconductor devices are required that are stable when stored under humid conditions, that is for example, at a temperature of about 70.degree. C, and at a relative humidity of over 90%, when stored for a period of several thousand hours. It has now been found that this requirement is satisfactorily fulfilled by the application of a protective layer consisting of a film of SiO.sub.2 containing 3% to 5% by weight of Al.sub.2 O.sub.3. However, many applications also require that the properties of such a protective layer do not change when the component is held for several thousand hours under "moisture-blocking" conditions (i.e. with similar or the same values of temperature and humidity, and with the application of a dc voltage in the inverse direction).
The simultaneous fulfillment of both requirements can indeed be obtained with an Al.sub.2 O.sub.3 content in the SiO.sub.2, which lies between the two values given above. It has been found, however, in order to obtain reproducible results, it is essential that the optimum Al.sub.2 O.sub.3 content in the SiO.sub.2 must be maintained with precision. Such precise control is relatively difficult and expensive to maintain.